In a multichannel receiver, a general problem relates to the calibration of the various reception channels in order to compensate for differences in time, phase and amplitude which may affect the signal received over the various channels. Specifically, each reception channel comprises an antenna and a radiofrequency reception chain which comprises, in particular, one or more shaping filters and one or more amplifiers. These various components might not have exactly the same characteristics; in particular, the group delays, the amplitude gain and the phase response may be substantially different. These differences lead to a mismatch of the reception channels, which should, in theory, all have the same frequency response (transfer function). The signals received over the various reception channels may thus be affected by these differences and the processing operations applied to the signals for the purpose of determining information on the positioning of the receiver or on the direction of arrival of the signals may be disrupted.
Within a context of applications that require highly reliable positioning information, it is therefore necessary to calibrate the receivers very precisely in order to minimize distortions between the various reception channels. This requirement is especially important for aeronautical applications or those linked to transport.
The known solutions for solving the problem of calibrating a multichannel GNSS receiver may be based on a factory pre-calibration carried out on each device and using measurements taken in a laboratory or in an anechoic chamber. As the response of RF reception channels is temperature-sensitive, it is necessary to use thermostatically controlled components or thermal sensors. Other solutions use a dedicated calibration radiofrequency signal which is generated by a specific device inside or outside the receiver.
In general, the existing solutions have the drawback of requiring additional equipment or devices.
Other solutions, described in the documents EP805510, U.S. Pat. Nos. 5,525,997 and 7,304,605, are also known.
The invention proposes a method for calibrating a multichannel GNSS receiver which does not require the use of a specific signal generator and which may be implemented directly on the basis of simple measurements taken from a receiver in operation.
The invention consists in particular of determining a first, broadband equalization filter which may be positioned at the output of the RF reception channels and at the input of the correlators in order to correct the mismatch between the various RF reception channels. The invention also consists of determining a second, narrowband equalization filter in order to correct residual phase and gain errors.
The invention is applicable to any satellite positioning system, in particular to compatible receivers of the GNSS GPS, GALILEO or GLONASS systems.